1. Field of the Invention
The present invention relates to a type of composite fine powder whose particles have metallic cores and ceramic surface layers, to a method for making it, and to an apparatus for making such fine powder, for practicing the method. In particular, the present invention relates to such a composite fine powder and to a method and an apparatus for making it, which is particularly suitable for utilization as reinforcement material for matrix metal in a particle dispersion type composite material or for use in making a sintered material, and in which a mixture of core metal in vapor form and a gas are rapidly cooled while also being combined together to form the ceramic outer layers of the particles by rapid expansion through a convergent-divergent nozzle.
2. Description of the Prior Art
Generally, ceramic type metallic compounds such as alumina, silicon nitride, tungsten carbide, and so on are far superior in heat resistance and wear resistance to metals in general; and accordingly it has often been attempted to construct various structural members of various apparatuses out of composite material in which particles of powder of such ceramics are dispersed in a matrix of metal, or alternatively of sintered material in which particles of powder of such ceramics are sintered together.
However, because powder particles consisting solely of such ceramics are very brittle, because the even dispersion of such ceramic powder particles in the body of the matrix metal is difficult, and because it is not always possible to ensure good contact between such ceramic powder particles and the matrix metal, such composite or sintered materials are not utilized on a wide scale at the present time, although they are used for some tool materials such as cermets.
Now, a solution to this which might be considered might be to form the reinforcing powder particles with metallic cores and ceramic surface layers, and this might overcome the problem of brittleness outlined above, but in practice in the past this has been very difficult. Performing surface treatment on metallic powder particles in order to provide them with ceramic outer layers has not been practicable for the production in any large volume of powder particles with average diameter of no more than a few microns. Now, of course, in the natural state powder particles of metals which have a strong tendency to become oxidized are covered with a layer of oxide on their surfaces, which is actually a ceramic, but since the typical thickness of such an oxide layer is only twenty angstroms or so in the case of aluminum for instance, or ten atomic layers at most, and since such a very thin oxide layer can be easily destroyed when force is applied to the powder particle, therefore the hardness of such particles and of the powder thereof as a whole is low, and such powder is quite insufficient in its properties as material for forming a powder reinforced type composite material or a sintered material.